Hepatitis C virus (HCV) is the major etiological agent of post-transfusion and community-acquired non-A non-B hepatitis worldwide. It is estimated that over 200 million people worldwide are infected by the virus. A high percentage of carriers become chronically infected and many progress to chronic liver disease, so-called chronic hepatitis C. This group is in turn at high risk for serious liver disease such as liver cirrhosis, hepatocellular carcinoma and terminal liver disease leading to death.
The mechanism by which HCV establishes viral persistence and causes a high rate of chronic liver disease has not been thoroughly elucidated. It is not known how HCV interacts with and evades the host immune system. In addition, the roles of cellular and humoral immune responses in protection against HCV infection and disease have yet to be established. Immunoglobulins have been reported for prophylaxis of transfusion-associated viral hepatitis, however, the Center for Disease Control does not presently recommend immunoglobulin treatment for this purpose. The lack of an effective protective immune response is hampering the development of a vaccine or adequate post-exposure prophylaxis measures, so in the near-term, hopes are firmly pinned on antiviral interventions.
Various clinical studies have been conducted with the goal of identifying pharmaceutical agents capable of effectively treating HCV infection in patients afflicted with chronic hepatitis C. These studies have involved the use of interferon-alpha, alone and in combination with other antiviral agents. Such studies have shown that a substantial number of the participants do not respond to these therapies, and of those that do respond favorably, a large proportion were found to relapse after termination of treatment.
Interferon in combination with ribavirin has been approved for the treatment of patients with chronic hepatitis C. However, side effects caused by IFN (such as retinopathy, thyroiditis, acute pancreatitis, depression) are not alleviated with this combination therapy. Pegylated forms of interferons such as PEG-Intron® and Pegasys® can apparently partially address these deleterious side effects but antiviral drugs still remain the avenue of choice for oral treatment of HCV.
Therefore, a need exists for the development of effective antiviral agents for treatment of HCV infection that overcome the limitations of existing pharmaceutical therapies.
HCV is an enveloped positive strand RNA virus in the Flaviviridae family. The single strand HCV RNA genome is approximately 9500 nucleotides in length and has a single open reading frame (ORF) encoding a single large polyprotein of about 3000 amino acids. In infected cells, this polyprotein is cleaved at multiple sites by cellular and viral proteases to produce the structural and non-structural (NS) proteins. In the case of HCV, the generation of mature nonstructural proteins (NS2, NS3, NS4A, NS4B, NS5A, and NS5B) is effected by two viral proteases. The first (generally referred to as the NS2/3 protease) cleaves at the NS2-NS3 junction; the second (the NS3 protease) is a serine protease contained within the N-terminal region of NS3 and mediates all the subsequent cleavages downstream of NS3, both in cis, at the NS3-NS4A cleavage site, and in trans, for the remaining NS4A-NS4B, NS4B-NS5A and NS5A-NS5B sites. The NS4A protein appears to serve multiple functions, acting as a cofactor for the NS3 protease and possibly assisting in the membrane localization of NS3 and other viral replicase components. The complex formation of the NS3 protease with NS4A seems necessary to the processing events, enhancing the proteolytic efficiency at all of the sites. The NS3 protein also exhibits nucleoside triphosphatase and RNA helicase activities. NS5B is a RNA-dependent RNA polymerase that is involved in the replication of HCV.
A general strategy for the development of antiviral agents is to inactivate virally encoded enzymes that are essential for the replication of the virus. In a two day clinical trial, it has been shown that the HCV NS3 protease inhibitor BILN 2061 is effective in rapidly reducing viral loads in patients infected with the hepatitis C virus (Gastroenterology (2004) 127(5): 1347-1355), thus providing proof of principle of the clinical antiviral activity of HCV NS3 protease inhibitors.
The NS3 protease has been found to potentially have an additional impact by blocking the IFN-mediated cellular antiviral activity in the infected cell (Foy et al., Science, 17 Apr. 2003). This lends credence to a hypothesis that the NS3/NS4A protease may represent a dual therapeutic target, the inhibition of which may both block viral replication and restore interferon response of HCV infected cells.
Inhibitors of the HCV NS3 protease have been described in WO 00/09543 (Boehringer Ingelheim), WO 03/064456 (Boehringer Ingelheim), WO 03/064416 (Boehringer Ingelheim), WO 02/060926 (Bristol-Myers Squibb), WO 03/053349 (Bristol-Myers Squibb), WO 03/099316 (Bristol-Myers Squibb), WO 03/099274 (Bristol-Myers Squibb), WO 2004/032827 (Bristol-Myers Squibb), and WO 2004/043339 (Bristol-Myers Squibb).